The present invention is directed generally to glass compositions, and more particularly to glass fiber compositions having high KI values and structural durability.
Glass fiber, or fiberglass, insulation is well known and has been a commercial product for many years. Glass fiber insulation is widely used both residentially and commercially.
Generally, the insulation is made from intertwined soda lime alumina borosilicate glass fibers which are held together with a binder. The glass fibers are generally produced using SiO.sub.2 with a number of additives, such as Na.sub.2 O, K.sub.2 O, CaO, MgO, BaO, B.sub.2 O.sub.3, and Al.sub.2 O.sub.3, that enhance various properties of fibers. The binder may be any suitable material, but quite commonly is a phenol-formaldehyde resin or a urea formaldehyde resin.
A rotary process is often used to form the glass fibers. The rotary process typically involves the introduction of molten glass into a rotating device, called a spinner, which contains a plurality of holes circumferentially distributed around the spinner. The spinner is rotated about an axis to produce a centrifugal force on the molten glass. The rotation of the spinner forces the molten glass through the plurality of holes.
An annular stream of hot gases is passed around the spinner to contact and attenuate the fibers passing through the holes. A spray nozzle is positioned to coat the attenuated fibers with the binder.
A conveyer collects the binder-coated fibers in the form of a blanket, or batt, and the blanket is heat cured to produce the final insulation. The rotary process can be used to produce insulation having different densities by varying the conveyer speed and the thickness of the cured insulation.
Glass fiber insulation has been used to replace, or in lieu of, asbestos fiber insulation in many applications. It is generally believed that asbestos fibers, when inhaled, can result in significant disease in man. Though the exact mechanism responsible for the biological activity of asbestos fibers is unknown, it is widely believed that an important factor in the mechanism is the residence time of the fibers in the lungs.
Unlike asbestos fibers, glass fibers have not been linked to disease in man. Glass fiber also appears to have a much shorter residence time in the lungs than asbestos fibers.
The residence time of glass fibers in the lungs will depend, at least in part, upon chemical dissolution of the fiber. The rate of chemical dissolution of a material in biological fluid is generally referred to as the biosolubility or biological degradability of the material.
Despite the lack of a link between glass fibers and human disease, some countries, for example Germany, have proposed regulations for the use of glass fibers in insulation products. Glass fiber compositions that meet the standard in the proposed regulations are considered to be free of suspicion as a disease causing agent and can be used for both commercial and residential installations.
The regulations are based on a desire to minimize the residence time of the glass fiber in the lungs. It is a hope that minimizing the residence time of the glass fiber will decrease the possibility, if any, of subsequent disease.
The proposed German regulations for biosolubility require that glass fibers have a numerical index (KI) greater than or equal to 40 to be considered to be free of suspicion. The KI index, which is sometimes referred to as the Wardenbach Index, is described by the equation: EQU KI=.SIGMA.(Na.sub.2 O, K.sub.2 O, CaO, MgO, BaO, B.sub.2 O.sub.3)-2(Al.sub.2 O.sub.3)
where the values for each oxide correspond to the weight percentage of that oxide in the glass composition.
The index used in the regulation places severe constraints on the compositions of the glass, expressly on the levels of alumina (Al.sub.2 O.sub.3) and implicitly on the level of silica (SiO.sub.2)in the glass composition. Manufacturers must now produce glass fibers which meet the proposed regulations, while maintaining standard performance criteria for the products. The criteria include that the glass fiber must be producible using standard wool processes, have sufficient durability in use, and acceptable insulating properties.
Silica is the primary component in glass fiber and provides most of the structural and physical properties of the fiber. Alumina is primarily used in the fiber to provide additional durability to the fiber.
Initial attempts to produce glass fiber that complies with the regulations involved using reduced levels of alumina in the glass composition to increase the KI index. However, low alumina glass fibers tend to have poor durabilities.
A number of glass composition have been reported as having improved biosolubility or biodegradability. For example, Potter, U.S. Pat. No. 5,055,428, Cohen et al., U.S. Pat. No. 5,108,957, Nyssen, U.S. Pat. No. 5,332,698, and Bauer et al., U.S. Pat. No. 5,401,693, all describe glass fibers having improved biosolubility. Also, published PCT applications WO 95/31411, WO 95/32925, WO 95/32926, WO 95/32927, and WO 95/35265 and numerous published German applications have reported glass compositions having increased biodegradability.
Despite the improvements presented in the aforementioned patents and applications, the glasses failed to meet the KI.gtoreq.40 standard or significant processing and performance problems remain. The decreased performances and increased processing costs for glass compositions designed to meet the new biological standards is a clear shortcoming in the industry. In addition, higher alumina compositions of the prior art provide performance versatility, yet are either not acceptable in the emerging regulated marketplace or suffer from increased processing costs. Accordingly, there is still a need for a glass composition which has increased biosolubilities (KI value.gtoreq.40), while possessing acceptable processing properties, such as viscosity and liquidus temperatures, as well as acceptable performance and durability in use.